Leishmaniasis is a major human health problem, and new immunotherapies are required to alleviate the chronic disease that is often associated with this infection. Infections of C57BL/6 mice with Leishmania mexicana fail to resolve, and therefore provide an excellent model to investigate the immune responses that lead to chronic human leishmaniasis. In this proposal, the mechanisms responsible for the meager immune response elicited by L. mexicana are investigated, focusing specifically on the inability of these parasites to induce a sustained expansion of the lymph node draining the site of infection. Lymph node expansion or hypertrophy is a physiologic response to infection, ensuring that the largest number of T cells come in contact with antigen-presenting dendritic cells. In contrast to L. mexicana, a related parasite, L. major, induces a healing infection in C57BL/6 mice and stimulates a rapid expansion in the size of the lymph node draining the lesion site. Dendritic cells have been shown to be critical for initiating lymph node hypertrophy, and studies to define what may be lacking in the dendritic cell response to L. mexicana are ongoing. One deficit uncovered was the inability of L. mexicana-infected dendritic cells to respond to immunostimulatory DNA or CpG. CpG activates dendritic cells by ligating TLR9, and corresponding with the deficit observed in L. mexicana-infected dendritic cells was the finding that L. major cannot induce lymph node hypertrophy in TLR9 deficient mice. This result indicates that TLR9 plays a previously unappreciated role in the early immune response to Leishmania. In Aim 1, studies are proposed to dissect the events mediating lymph node hypertrophy to understand why L. mexicana fails to induce this response. In Aim 2, the mechanisms associated with the inability of L. mexicana-infected dendritic cells to respond to CpG will be investigated. Finally, in Aim 3, studies to promote increased lymph node expansion, using dendritic cells and TLR ligands, are proposed. Taken together, these experiments should provide the foundation for the development of new immunotherapies for chronic leishmaniasis.